A wonderful optical illusion

I have heard about these kinds of illusions before, but never seen one actually in front of me. This one was sent to me by my friend Rob. Click here to see a movie that demonstrates how it looks.

And click here to see how to actually build one, which you absolutely should!

After I built it, nothing happened: the illusion would not “snap in”. I stared at it for a while with only one eye open, and then it happened. When it does, it is really amazing. And I was able to look at it with both eyes after it “snapped in”.